Carp Helped Kill Lake Austin's Hydrilla, But What Are They Eating Now?

Jul 23, 2015

In the past, hydrilla carpeted whole swaths of Lake Austin. The invasive plant ruined recreation and damaged ecosystems on the lake. So to counteract that, the City of Austin occasionally introduced tens of thousands of sterilized grass carp to eat the hydrilla. But the city is now on the lookout for unintended consequences.

You’ve got to hand it to the grass carp: They did their job swimmingly. There’s no hydrilla problem in the lake right now, but there is concern the thousands of hungry fish have turned their attention to native plant species, and even other fish.

“Yeah, some of the anglers have talked about while they’re off fishing that they’re actually able to catch grass carp on crank baits. So, that’s what really got their hackles up,” says Dr. Brent Bellinger, an environmental scientist with the Watershed Protection Department. “Well, if they’re going after something that looks like shad on crank baits, they might be going after shad in general.”

The carp are known as “triploid” grass carp – they have a third set of chromosomes that limit their viability. So, ideally, they can’t reproduce at all, unlike diploid carp. The city promises on its website that, unlike in “Jurassic Park,” nature won’t find a way to allow triploid carp to reproduce.

Still, Bellinger wants to figure out exactly what the carp are now eating and how long they’ll stay in the lake. To that end, the city partnered with Texas Parks and Wildlife and area anglers to pull about 200 carp out of the lake. He says there was no visual evidence of other fish in the carps’ stomachs.

“As far as we can tell, no. We’re going to do some additional biochemical analysis to try and confirm that,” Bellinger says.

As far as native plants go, the city is trying to create special habitats to encourage their regrowth in the lake. Bellinger expects the sterilized carp will die off in time. A commonly cited study from 1986 estimated their life spans at 10 to 15 years, and found that out of 250,000 embryos, only a few survived more than a week after hatching. While none of the fish survived a month after hatching, some scientists are still running tests on the recently captured fish to see if those estimates were correct.

A non-native aquatic plant, hydrilla spread rapidly in Lake Austin since it was first discovered in 1999. It grows to be very thick and can clog up pipes that carry drinking water from the lake. It can also cause problems for those who use the lake for recreation.

At one point in 2012, it covered a record 580 acres of Lake Austin.